The present invention relates to indicator systems for indicating time-temperature histories of a product. More particularly, it relates to a new and improved time-temperature integrating indicator for monitoring the safe limits of refrigerated storage for a food product or other material.
The desirability of detecting whether or not a food product has been subjected to adverse temperature conditions has been recognized and numerous indicator devices of this kind are described in the patent literature. One especially prevalent class of these indicators relates to indicators for detecting whether or not a frozen food product has been allowed to thaw. Typically, these freeze indicators include a frozen material which melts at some preselected temperature so as to irreversibly activate an indicator, either chemically or physically. Typical of these devices are those described in U.S. Pat. No. 3,437,010 and elsewhere. Most of the frozen food indicator devices merely signal a thaw, without making any attempt to measure the period during which the product is thawed or the temperature which the product attains while it is or was thawed.
Another class of known indicators for food products utilizes diffusion or capillary action with a wick or similar permeable member to provide some degree of gradation such as those shown in U.S. Pat. Nos. 3,414,415 and 3,479,877 to name but a few.
It is now recognized that various natural and synthetic materials deteriorate with the passage of time, even when taking such precautions as storing under refrigeration, packaging in an inert atmosphere, sterilization and adding spoilage retardants. For example, foods, film, pharmaceuticals, biological preparations and the like may each demonstrate decomposition with the passage of time, even when sterilized or maintained at sufficiently low temperature to preclude microbiological degradation. Decomposition may occur for various reasons including strictly chemical reactions such as oxidation and enzymatic processes. Consequently, for each type of material there often exists a limit to its permissible storage life, after which time a discernible change in some property occurs. A system which indicates when this time limit has been exceeded is desired in the food packaging industry.
The deterioration kinetics involved in these processes may be exceedingly complicated. Generally, deterioration is a function of temperature, however, the rate of deterioration of a product may vary with the temperature, so that one rate of deterioration is observed at a first temperature and a different rate of deterioration is observed at a second temperature. The overall or total amount of deterioration will depend upon the time at which the product is held at each temperature, i.e., an integral of time and temperature. The practical effects of this can be seen, for example, from two identical samples of food packaged simultaneously. For both of these there may be a finite time temperature integral until a discernible change in food quality occurs. If one package is allowed to rise in temperature by 10.degree. or 20.degree. or more in the course of its distribution or storage, its quality shelf life will be reduced as compared with the other package which is maintained at an appropriate lower refrigerated temperature for its entire storage life. Consumers, about to purchase these packages, both of which are now stored at a normal refrigeration display case presently have no way of ascertaining this difference in the temperature histories of the products they are buying.
Systems have been suggested for monitoring the temperature history of a product. U.S. Pat. No. 2,671,028 utilizes an enzyme such as pepsin in indicator systems. U.S. Pat. No. 3,751,382 discloses an enzymatic indicator in which urease decomposes urea with the reaction products causing a change in the pH of the system. The activity of the enzyme and thus the rate of decomposition is dependent on temperature, so that the change in pH resulting from the activity of the enzyme can be monitored by conventional acid-base indicators. Another system is described in U.S. Pat. No. 3,768,976, wherein time temperature integration is achieved by monitoring oxygen permeation through a film through the use of a redox dye.
Finally in U.S. Pat. No. 3,942,467, a time-temperature integrating indicator is described including an organic acid generating component which is subject to solvolysis at a first reaction rate to generate known amounts of acid. The solvent or solvolysis medium is provided with known quantities of alkaline materials sufficient to neutralize acid generated by solvolysis for a given time period at a given reaction temperature. The solvolytic reaction rate increases with an increase in temperature. A pH sensitive dye is provided to indicate when a sufficient amount of acid has been generated to neutralize all of the alkaline material. In a preferred embodiment, two separate alkaline materials of different basicities are employed in combination with more than one pH sensitive dye. In accordance with this preferred embodiment, the indicating solution indicates a first color until the first alkaline species has been neutralized to an extent sufficient to decrease the pH of the indicator to a first intermediate range, thereby causing a color change to a second intermediate color. The second color remains until the second alkaline material is neutralized, shifting the indicator pH into the acid range, whereupon the overall indicator changes to a third color.
In accordance with this prior art indicator, gradual changes in color are observed and time delays in generating a discernible color change may take longer than is desirable or required for certain indicator applications. Moreover, as is evident from the patent, the solvolysis of the acid precursor materials requires the use of special solvent combinations to increase the half life of the solvolytic reactions. This is undesirable in today's context, wherein the co-solvents suggested in the patent include, for example, dioxane. These co-solvents are generally to be used sparingly.
A basic problem in developing satisfactory time-temperature integrating indicators is the fact that the second derivative of time-temperature decomposition (the change in rate per unit of temperature change), differs for different products. Thus, the change in the rate of deterioration per unit of temperature change for certain fruits and berries is vastly different from the change in rate for lean meats. The values for dairy products are different from both berries and meats. Consequently, a system which is dependent on a single enzymatic reaction or the permeability of a given film would be suitable as an indicator only for those materials having a similar slope for the relationship of change of rate of decomposition of a certain kind with respect to temperature.
Time temperature integrating indicator systems are not limited in application to monitoring long storage periods at below room temperature. The same considerations apply to shorter time periods and to high temperatures. The systems can be used to ensure, for example, that products have been adequately heat sterilized. The indicator is thus admirably suited to ensure that canned goods which are autoclaved have been subjected to the appropriate time and temperature integral required to obtain a necessary degree of microorganism kill. In this case, the firing of the indicator is used as the signal that the necessary sterilizing parameters have been reached or exceeded. Similarly, indicators of this type can be used to ensure that surgical instruments have been subjected to appropriate sterilization conditions, or that pharmaceuticals have not been stored for periods in excess of that which is permissible. Similarly, indicators of this type are useful in the dairy industry to indicate that dairy products have been properly Pasteurized and the like.
A major problem with prior art time temperature integrating indicators is that their time response is generally unacceptable. Frequently, the development of sufficient color to be differentiable by the viewer, may take as long as 30 to 100% of the life of the indicator.
At present, with the advent of modified atmosphere packages and newer, safer preservative species for extending the room temperature shelf life of cooked fresh foods, the consuming public has come to enjoy a number of high quality fresh tasting food products which heretofore were unavailable. Instead of providing the food in frozen form or in a can, cooked fresh foods may be treated and stored in modified atmosphere packages and stored for sufficiently long periods under refrigerated conditions to now permit these fresher foods to be available to the consumer. Luncheon trays including a variety of luncheon meats, such as turkey and ham and cheeses are an illustrative example. The improvements making these products possible have now extended the room temperature storage stability of the products in these packages from a matter of hours to a day, to several days to a week. This permits the distribution chain for the food products to make the food products available to the consumer for immediate consumption within their improved and extended shelf life.
In theory, the ratio of the rate of change at one temperature of a property of a stored article whose deterioration is being monitored to the rate of change at a lower temperature differs for different materials. This value is often expressed for 10.degree. increments and is represented by the symbol Q.sub.10 for the Celsius scale and a.sub.10 for the Fahrenheit scale. For example, within the range of 0.degree. to -20.degree. C. raw fatty meat and precooked fatty meat have rancidity Q10 values of about 3, whereas, raw lean meat and precooked lean meat have rancidity Q.sub.10 values of between 5 and 6. Vegetables generally have a spoiling Q.sub.10 of between 7 and 8, whereas fruits and berries have a spoiling Q.sub.10 of approximately 13. The formulation of components of any indicator system should be selected so that the change in the rate of development of signal per unit change in temperature should be compatible with the parameters desired in the article being monitored. The Q.sub.10 value for the indicator generally should approximate the Q.sub.10 value of a property of a given food class being measured. To make this match, the proper selection of the indicator ingredients and appropriate manipulation of the relative concentration of these ingredients needs to be performed generally in accordance with the methods described in U.S. Pat. No. 3,942,467, the teachings of which are specifically incorporated herein by reference.
Accordingly, it is an object of the present invention to provide a new and improved time temperature integrating indicator for use with food products adapted for room temperature or refrigerated storage which provides an immediate visual indication of product safety and quality.
It is another object of the present invention to provide a time temperature integrating indicator for use with foodstuffs which rapidly develops a distinct end point signal as soon as safe storage limits have been reached.
It is a further object of the present invention to provide a new and improved time temperature integrating indicator which may be used with a wide variety of foodstuffs and in a wide variety of packaging configurations.